The present invention relates to thermostatically operated switches for controlling cooling or heating equipment where the switch is actuated and deactuated in response to sensing a temperature rising and falling through predetermined levels. In particular, the present invention relates to thermostats of the type having a fluid pressure capsule which expands and contracts responsive to temperature changes, for actuating a snap-acting switch for making and breaking a set of electrical contacts in a circuit for controlling desired heating or cooling functions. Thermostats of this type usually have a sensing bulb disposed remotely from the switch mechanism and connected to the fluid pressure capsule by a capillary tube for transmitting fluid pressure to the capsule in response to temperature changes sensed by the remote fluid filled bulb.
In fluid filled bulb-and-capillary thermostats of the above described type, it has long been desired to find or way or means of providing for switch actuation in the event of loss of fluid in the bulb and capillary system resulting in total depressurization of the pressure capsule.
Typically, the pressure capsule is calibrated at room temperature to provide switch actuation upon a desired sensed rise in temperature and to deactuate when the temperature drops a desired amount from the actuation point. However, if the pressure capsule integrity is breached and fluid loss occurs, the system is rendered totally inoperative and extreme temperatures may be encountered at the bulb with no resultant change in system control. Accordingly, it has been desired to provide a way of producing switch actuation upon the pressure capsule experiencing lost fluid therein for the purpose of providing a signal input to the system being controlled indicating that a failure in the sensor mechanism has occurred.